1. Field of the Invention
The present invention relates generally to a digital Real Time Clock (RTC) monitor in a Global Navigational Satellite System (GNSS) receiver and having a single pin for both power-on reset and wake-up interrupt signaling between chips, and, more particularly, to a digital RTC monitor that continually assesses whether an RTC oscillator has missing cycles, and to having a single wire connecting two or more chips for power-on reset and wake-up interrupt request signaling between them.
2. Description of the Related Art
Satellite navigational systems provide positional and timing information to earth-bound receivers. Each system has its own constellation of satellites orbiting the Earth, and, in order to calculate its position, a receiver on Earth uses the satellites “in view” (i.e., in the sky above) from that system's constellation. Global Navigational Satellite System (GNSS) is often used as the generic term for such a system, even though such navigational satellite systems include regional and augmented systems—i.e., systems that are not truly “global.” The term “GNSS,” as used herein, covers any type of navigational satellite system, global, regional, augmented or otherwise, unless expressly indicated otherwise.
The number of GNSS systems, both planned and presently operational, is growing. The widely-known, widely-used, and truly global Global Positioning System (GPS) of the United States has been joined by one other global system, Russia's GLObalnaya NAvigatsionnaya Sputnikovaya Sistema (GLONASS), and is presently being joined by Europe's Galileo and China's BeiDou (also known, in its second generation, as COMPASS) systems—each of which has, or will have, its own constellation of satellites orbiting the globe. Regional systems (those that are not global, but intended to cover only a certain region of the globe) include Japan's Quasi-Zenith Satellite System (QZSS) and the Indian Regional Navigational Satellite System (IRNSS) currently being developed. Augmented systems are normally regional as well, and “augment” existing GNSS systems with, e.g., messages from ground-based stations and/or additional navigational aids. These include the Wide Area Augmentation System (WAAS), European Geostationary Navigation Overlay Service (EGNOS), Multi-functional Satellite Augmentation System (MSAS), and GPS Aided Geo Augmented Navigation (GAGAN). Regional GNSS systems, such as QZSS, can also operate as augmented systems.
Moreover, GNSS capabilities are no longer limited to any particular type of system or device. A GNSS receiver may be implemented in a mobile terminal, a tablet computer, a camera, a portable music player, and a myriad of other portable and/or mobile personal consumer devices, as well as integrated into larger devices and/or systems, such as the electronics of a vehicle. The term “GNSS receiver” as used herein, covers any such implementation of GNSS capabilities in a device or system.
An accurate “clock” is essential for GNSS receiver performance, and the various GNSS functions, e.g., acquisition, tracking, positional computations, etc., rely on maintaining accurate timing to a greater extent than most other functions of electronic devices (accuracy within ±0.05 ms). Thus, a GNSS receiver usually has its own GNSS oscillator, regardless of what system or device it is a part of. However, GNSS oscillators also use much more power than other oscillators. Because of this power usage and the power usage of all of the other GNSS components, e.g., the reception chain, the acquisition, tracking, and computation components, etc., most portable devices having a GNSS receiver turn off one or more GNSS components when the GNSS function is not being used.
Since the GNSS oscillator is also turned off, a cold, warm, or hot restart of the GNSS receiver may be required to obtain accurate measurements when the GNSS receiver is turned on again. However, if the shut down period has been sufficiently short (and/or the user's location has not changed significantly), it is possible to reacquire the previous satellite signals and achieve nearly immediate correlation of the GNSS signals (rather than the several seconds to minutes associated with the hot, warm or cold start procedures). Nearly immediate correlation saves several seconds, thereby saving a substantial amount of the limited power available in a portable GPS receiver unit.
Such nearly immediate (re-)correlation requires keeping time during the period the GNSS oscillator is off. Typically, a Real Time Clock (RTC) circuit is used to maintain rough GNSS time while the rest of the GNSS circuitry is off. Typical RTC circuits are low cost and have poor stability and temperature characteristics. Thus, while they may maintain accuracy within a few seconds over extended periods, which is adequate for hot and warm starts, over shorter periods, the RTC may or may not keep within the required ±0.05 ms, so it is not clear whether immediate (re-)correlation or a warm/hot start is required after such a short off period. Moreover, the RTC may stop due to partial or total loss of power, experience a brownout condition, or miss cycles for some other reason, during the off period. In such circumstances, the GNSS receiver will need to do a cold start, regardless of how long it was off. But the GNSS receiver needs to know if such a problem occurred while it was asleep/off.
Some portable devices having a GNSS receiver use an RTC monitor in the form of analog circuitry to determine if such an RTC problem occurred while the GNSS receiver was off/asleep. However, such analog RTC monitors, which are always on, require power, space, and other limited resources of the portable device. On the other hand, when the power-on reset and wakeup interrupt request functions are shared between two chips, two dedicated wires must be used if the functions must work when the general purpose processors are sleeping, clock-stopped, or otherwise disabled.
Thus, methods, systems, and apparatuses are needed for low-power and efficient RTC monitoring while the GNSS receiver is off, sleeping, and/or otherwise disabled and for providing a single line for power-on reset and wakeup interrupt request functions between two chips.